This invention relates to an improvement in the method and apparatus for making continuous metal filaments, particularly filaments of metal alloys having amorphous molecular structure, by depositing molten metal onto the peripheral surface of a rotating annular chill roll to form a solid filament thereon. The improvement effects prolonged contact between the chill roll and the solidified filament past the point where it would separate from the chill roll by action of centrifugal force.
For purposes of the present invention, a filament is a slender body whose transverse dimensions are much less than its length. In that context, filaments may be bodies such as ribbons, strips, sheets or wires, of regular or irregular cross section. As used herein, an "amorphous filament" is a filament of a metal alloy having amorphous molecular structure.
It is known to make metal filaments by directing a jet of molten metal against a rotating quench surface, or by otherwise depositing molten metal on such quench surface, whereon it is solidified in the form of a ribbon and is flung away by action of centrifugal force, as for example described by Strange and Pim in U.S. Pat. No. 905,758. In the procedure described by Strange and Pim the quench surface is furnished by a rotating chill roll. That procedure is suitable to form filaments of many of the polycrystalline metals which possess sharp melting points, that is to say, which have solid-liquid transition range of less than about 4.degree. C. However, glassy metals having amorphous molecular structure often have a transition range in the order of about 400.degree. C. or more through which the viscosity of the metal gradually increases until the critical glass transition temperature is reached, and it is necessary for the filament to be quenched below its glass transition temperature before departure from the quench surface. This is difficult to achieve by the procedure of Strange and Pim because centrifugal action tends prematurely to fling the filament away from the chill roll. Also, in that procedure the point of release of the filament from the surface of the chill roll varies, so that it is difficult to collect the filament and to guide it to a suitable winder.
Shortcomings concerning insufficiency of retention time of filament on the surface of the chill roll, and difficulties in collecting the filament from a variable point of release, are overcome by the procedures described by Kavesh in U.S. Pat. No. 3,856,074 and Bedell in U.S. Pat. No. 3,862,658. The Kavesh procedure involves detection of filaments formed on the exterior surface of a rotating chill roll by use of nipping means; the Bedell procedure involves prolonging the period of contact between the filament and the chill roll by exerting a force against the surface of the chill roll in the direction towards the axis of rotation of the chill roll by devices such as gas jets, moving belts and rotating wheels. While highly effective, the procedures of Kavesh and Bedell inherently involve danger of damaging the highly sensitive surface of the chill roll, as by marring it because of solid-to-solid contact between the surface of the chill roll and the nipping means, moving belts or rotating wheels, or because of accidental impingement of debris on the surface of the chill roll by the gas jets. The slightest imperfection in the surface of the chill roll will be noticeable on the surface of the filament formed, resulting in a product of inferior appearance.
A further problem inherent in chill roll casting of filament is that the point at which the filament is released and flung away from the surface of the rotating chill roll by action of centrifugal force is unstable, due to variations in adhesion between the filament and the chill roll surface. Stewart et al. in U.S. Pat. No. 3,812,901 disclose a procedure to stabilize the point of release involving supporting the filament formed on a rotating chill roll on a support member below the free flight trajectory of the filament, and applying a tension force to the filament such that it leaves the rotating chill roll without the aid of the applied tension. The magnitude of the applied tension is less than other forces causing release of the filament from the rotating chill roll, and is sufficient only to overcome variations in adhesion of the filament to the rotating chill roll, with the filament spontaneously releasing from the chill roll, with or without the application of the tension force. Stewart et al.'s method thus has the effect only of stabilizing the point of departure, but it does not prolong contact between a chill roll and the filament.
It is an object of the present invention to provide a method for obtaining controlled retention of metal filament formed on the peripheral surface of a rotating chill roll.
It is another object of the present invention to provide a method and apparatus for controlled retention of filament formed on the peripheral surface of a rotating chill roll, which avoids danger of damage to the surface of the chill roll.